TRANSPIRATION PHYSICO-CHEMICAL · TRANSPIRATION AND PHYSICO-CHEMICAL PROPERTIES OF LEAVES AS...

TRANSPIRATION AND PHYSICO-CHEMICAL PROPERTIES OFLEAVES AS RELATED TO DROUGHT RESISTANCE IN

LOBLOLLY PINE AND SHORTLEAF PINE'

C. S. SC HOPM EYER

(WITH TWO FIGURES)

Introduction

In the reforestation program now being carried on by various state andfederal agencies, many millions of tree seedlings are planted each year. Anextremely large number of these planted seedlings are killed within the firstyear or two after planting. One of the major causes of the fatalities is theinability of seedlings to resist drought. If the properties involved indrought resistance of seedlings were known, and if those species and ageclasses of planting stock possessing such properties could be identified ordeveloped, much better survival could be obtained in plantations of treeseedlings.

Drought resistance, the ability of a plant to endure permanent wiltingwithout harm to subsequent development (11), has been induced in plantsby repeated wilting and also by growing them in soil with a moisture contentbarely above the wilting percentage (27, 28). Observations on permanentlywilted herbaceous plants (27) show that their transpiration rate may beless than one-fourth of the rate for turgid plants. Most conifers, however,do not show any external evidence of a wilted condition. Hence, in harden-ing conifers against drought, the wilting percentage of soil moisture must bepreviously determined to facilitate control of soil moisture conditions.

Although attempts have been made to correlate drought resistance withrate of transpiration, there is much evidence (1, 2, 4, 21, 24, 32) to supportthe statement of MAXIMOV anld KRASNOSSELSKY-MAXIMOV (12) that the rateof transpiration when soil moisture is abundant cannot be used for judgingdrought resistance. Transpiration may be greatly reduced when soil mois-ture approaches the wilting percentage (15, 19) and the reduction may varyamong plants of different morphological structure, but there appears to beno consistent relationship between drought resistance and rate of transpira-tioni at soil moisture contents near the wilting percentage.

There is evidence of a correlation between drought resistance and boundwater content of leaf tissue. KORSTIAN (9) found some indication of such acorrelationi in a preliminary study of southeastern tree species. The workof NEWTON and MARTIN (17) is particularly outstanding in that a verydefinite correlation was obtained in cereal crops and grasses between the de-gree of drought resistance and the bound water content. NovIKOV (18) also

1 This investigation was supported by a Fellowship in Forestry in the Graduate Schoolof Arts and Sciences of Duke University.

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PLANT PHYSIOLOGY

found that the amount of bound water present characterized the degree ofdrought resistance in varieties of wheat when they were in a wilted state butnot in a turgid state. He attributed an observed increase in bound waterduring hardening not to a decrease in total water but to physico-chemicalchanges in the cell contents.

Information on the relationship of osmotic pressure of cell sap to droughtresistance is very conflicting. DRABBLE and DRABBLE (3) and LIVINGSTON(10) have shown that a high osmotic pressure cannot lower the vapor pres-sure of cell sap sufficiently to reduce water loss by transpiration to an appre-ciable extent. KORSTIAN (8) obtained evidence indicating that water ab-sorption by plants depends on osmotic pressure, but STODDART (25) believesthat a high osmotic pressure is a result of drought and not an adaptation toit. NEWTON and MARTIN (17) found that osmotic pressure was not a con-sistently reliable index of drought resistance. In comparing osmotic pres-sures of two or more species it is difficult to make a proper interpretationbecause in most cases it is not known whether differences are caused by dif-ferent water contents or by different solute concentrations.

The primary purpose of this investigation was to ascertain whether therelative drought resistance of seedlings of two species can be determined byphysico-chemical means. A secondary purpose was to obtain information onthe behavior of seedlings during and after drought by a study of changes inphysico-chemical properties. The approach was made by a study of trans-piration, bound-water content, total water content, osmotic pressure, andcalculated solute concentration in leaves of two species growing in a green-house under controlled soil moisture conditions. Loblolly pine (Pinus taedaL.) and shortleaf pine (P. echinata Mill.) were selected for comparison be-cause both species occur within the same region with loblolly pine on mesicsites, and shortleaf pine on more xeric sites. The apparent adaptations ofthese species to different sites indicate that shortleaf pine is the moredrought resistant of the two species.

Materials and methods

SEEDLINGS AND SOIL

A group of vigorous one-year-old seedlings of loblolly pine and of short-leaf pine were selected at the North Carolina State Nursery and moved to agreenhouse in December, 1935. The seedlings were planted in two-gallonbuckets containing Congaree silt loam, the moisture constants of which arepresented in table I. A watering tube extended down the side of eachbucket into a layer of crushed rock in the bottom. The buckets were coveredwith a double layer of oilcloth to prevent evaporation of water from thesoil. Soil moisture was maintained at 30 per cent. for a month after thecompletion of planting to permit the seedlings to become established.

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SCHOPMEYER: DROUGHT RESISTANCE IN CERTAIN PINES

TABLE IMOISTURE CONSTANTS FOR CONGAREE SILT LOAM

ITEM MOISTURE CONTENT(PERCENTAGE OF DRY WEIGHT)

Moisture equivalent ................ .................. 27.3Wilting coefficient .............. .................... 14.8Wilting percentage ................ .................. 7.7Saturation percentage .................... .............. 53.3

DETERMINATION OF TRANSPIRATION

The gravimetric method for the determination of transpiration was se-lected for this work. The buckets were weighed daily to determine theweight of water lost by transpiration which was expressed in grams per gramof oven-dried leaf tissue per day.

On April 4 the seedlings of each species were divided into three groupsfor a study of transpiration under three conditions of soil moisture. In thefirst group of seedlings, the soil moisture was brought to 30 per cent. atweekly intervals. This moisture content is believed to be approximately anoptimum for growth. Between waterings the soil moisture in this grouprarely went below 25 per cent. In the second group, the seedlings were leftunwatered, and the soil moisture was gradually decreased to the wilting co-efficient. Approximately six weeks elapsed before the wilting coefficient wasreached. This treatment, according to TUMANOV (27), should inducedrought hardening. In the third group, the soil moisture was allowed toreach the wilting coefficient, as in the second group, and was then broughtback to 30 per cent. moisture. The latter treatment enabled the seedlings torecover from the effects of drought and to resume growth. In the first twogroups, transpiration determinations were made at weekly intervals. Inaddition, daily determinations were made near the end of the experimentsfor all three groups.

DETERMINATION OF PHYSICO-CHEMICAL PROPERTIES

After completing the transpiration determinations ten samples were col-lected from each species in each of the three soil moisture categories. Col-lections were made at 11 P.M. One sample consisted of all the leaves fromone seedling. The fresh weight of the sample was obtained after placingit in a closed tin container. Each sample was macerated in a meat grinderand divided into three parts for the determination of bound water, totalwater, and freezing point, respectively.

The calorimetric method for the determination of bound water (5, 13, 16,20, 22, 23, 26) was selected for this work. In making the determinations, ten

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4PLANT PHYSIOLOGY

samples from each category were run through the procedure together. Thesamples, each consisting of about 8 grams of the macerated leaf tissue, wereplaced in tinfoil cups similar to those used by ROBINSON (20) and GREAT-HOUSE (5). The cups were placed in glass vials, tightly stoppered, andweighed to + 0.001 gram. After weighing, the tinfoil cups containing thesamples were placed in stoppered freezing tubes with two samples in eachtube. The freezing tubes were suspended in a freezing bath at a tempera-ture of - 200 C. ± 20 C. through holes in a cover over the bath. The sampleswere left in the bath for approximately 12 hours. A pint thermos bottle,containing 250.0 gm. of water was used for a calorimeter. The initial tem-perature of the water in the calorimeter was determined by using a mercurythermometer which was read to ± 0.0050 C. A sample, enclosed in its tin-foil container, was transferred from the freezing bath to the calorimeter.After continuous stirring with a motor-driven stirring rod for approxi-mately 8 minutes, thermal equilibrium was reached and the final tempera-ture was recorded. A correction factor for the heat absorbed by the calo-rimeter and accessories was obtained by placing 8 ml. of water in the tinfoilcups and following the procedure just outlined. The formulae used in cal-culating the bound water contents are essentially the same as those used byGREATHOUSE (5).

In preparing samples for specific heat determinations on the dry matterin the leaf tissue, leaves were dried for 24 hours at 950 C. and then groundto a fine dust with a mortar and pestle. The ground leaf tissue was placedin tinfoil cups and dried to constant weight ± 0.001 gram at 950 C. Thespecific heats were determined using the same procedure as for bound waterexcept that benzene was used instead of water in the calorimeter. Benzene,having a lower specific heat, facilitated measurement of the temperaturechange in the calorimeter. The specific heats of benzene and of tin wereobtained by plotting their values as given in the International Critical Tables(31) and then interpolating for the desired temperature. The specific heatsof water and of ice were obtained from the Handbook of Chemistry andPhysics (7). For loblolly pine the mean of 5 specific heat determinationson the dry leaf tissue was 0.213 with a standard error of -+- 0.002; the meanof 5 determinations on shortleaf pine was 0.208 with a standard error of-0.006.The total water content of a sample was assumed to be the difference

between the fresh weight and the weight after drying the macerated tissueto constant weight + 0.001 gm. at 950 C. The water content was calculatedon a dry weight basis.

Freezing point determinations were made on the leaf tissue using thethermo-electric method (14). The apparatus consisted of two thermo-couples connected in series to a galvanometer. One junction was maintainedat 00 C. The other junction was sealed into the tip of a hypodermic needle

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SCHOPMEYER: DROUGHT RESISTANCE IN CERTAIN PINES 451

and inserted into a 1.5-cm. length of glass tubing containing a portion ofthe macerated leaf tissue. The junction with the enclosing sample was

placed in a freezing tube suspended in a freezing bath. The amount ofundercooling and the observed freezing point were recorded. The true freez-ing point and the osmotic pressure were calculated from the tables of HARRISand GORTNER (6).

Since osmotic pressure of a tissue varies with total water content, the use-

fulness of osmotic pressure for comparisons of solute concentration in tissuesof different water contents is very limited. In an attempt to obtain a more

stable factor for such comparisons, the following conversion was devised.rrx [H120] = [Solute]

22.4

whereT = osmotic pressure

[H20] = grams of water per gram of dry leaf tissue

[Solute] = number of gram molecular weights (mols) of solute per kilogramof dry leaf tissue

22.4 = the osmotic pressure of a mol of undissociated solute in 1000grams of water

This conversion does not give a true measure of solute concentration becausethe relative amounts of ionized and un-ionized solutes are not kniown. Thisformula merely converts osmotic pressure to its equivalent in mols of undis-sociated solute on a dry weight basis and thus gives a factor which probablydoes not vary to a great extent with changes in water content. This factorwill be designated as solutte concentration in the following discussion, but

4-

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16

Time (days)

FIG. 1. Daily transpiration of loblolly pine and shortleaf pine at 30 per cent. soilmoisture and at the wilting coefficient. Each point is the mean value of 9 determinations.

ShoriotPt*at wilting coefficient

__ __. -F-e-4----2 3 4 5 6 7



PLANT PHYSIOLOGY

actually it is just a rough approximation to the true solute concentrationbecause of the unknown degree of dissociation.

Results and discussion

TRANSPIRATION

Shortleaf pine had a higher daily rate of transpiration than loblolly pinewith soil moisture at 30 per cent. and also at the wilting coefficient. Meanvalues of nine trees of each species in the two soil moisture categories onseven successive days are shown in figure 1. The differences between thetwo species are highly significant at both soil moisture contents as shown bythe analysis of variance in table II. Significant also is the variation in

TABLE IIANALYSIS OF VARIANCE IN TRANSPIRATION RATES OF LOBLOLLY PINE AND SHORTLEAF PINE

SOURCE OF DEGREES OF SUM OF MEAN CALCULATEDVARIATION FREEDOM SQUARES SQUARE F VALUE

Soil moisture at 30 per cent.

Species ...1...;...... 222.60 222.60 59.52tDays .............................6 915.56 152.59 40.80 tInteraction..6 45.30 7.55 2.02*Error ................... 112 419.35 3.74Total ...... .. 125 1602.81

Soil moisture at wilting coefficient (14.8 per cent.)

Species ...........1 41.18 41.18 76.26tDays ................6 16.99 2.83 5.24tInteraction 6 16.06 2.68 4.96tError ..................... 112 60.27 0.54Total .. ..... 125 134.50

* Not significant (less than 5 per cent. point).t Highly significant (greater than 1 per cent. point).

transpiration rates resulting from the variation caused by differences intemperature and humidity from day to day. These results indicate thatthe apparently greater ability of shortleaf pine to resist drought cannot beattributed to an ability to conserve water by retarding transpiration. Thisconclusion agrees with the statement of MA:XIMOV (11) that xeric plants aredistinguished by a higher rate of transpiration than that of more mesicplants.

The rate of transpiration of loblolly pine gradually diminished over aperiod of six weeks as the decreasing soil moisture approached and passedthe wilting coefficient (14.8 per cent. moisture). At the end of the six-weekperiod, the rate was only 15.8 per cent. of that at 30 per cent. soil moisture.This gradual reduction is not to be expected according to the results ofVIEHMEYER and HENDRICKSON (30). They found that extraction of water

452




from soil by peach trees is not affected by soil moisture content until it isreduced to the wilting percentage. Hence transpiration would be expectedto remain constant until the wilting percentage is reached. The gradualreductioni observed in this work can be explained, however, by assuming thatat the beginning of the period of drought, only a portion of the roots were insoil at the wilting coefficient; and as more and more of the roots extracted allof the water available to them, transpiration was gradually reduced. Thetranspiration rates of the two species at the wilting coefficient, shown infigure 1, are not as low as the negligible amounts of water lost by westernconifers as observed by PEARSON (19). The calculated wilting coefficient forthe soil used in this study, however, as shown in table I, is approximately 7per cent. higher than the true wilting percentage as determined experi-mentally. Hence there may have been an appreciable amount of availablesoil moisture at the calculated wiltinig coefficient.

With soil moisture at 30 per cent. after having been reduced to the wilt-ing coefficient, differences in transpiration rates between the two specieswere not significant. A comparison of the transpiration rates of loblolly

TABLE IIIANALYSIS OF VARIANCE IN BOUND WATER, TOTAL WATER, OSMIOTIC PRESSURE, AND SOLUTE

CONCENTRATION IN LEAF SAMPLES OF LOBLOLLY PINE AND SHORTLEAF PINECOLLECTED AT 11 P. M.

SOURCE OF DEGREES SUM OF MEAN CALCULATEDVARIATION

oSQUARES SQUARE F VALUEFREEDOM

Species 1 109.54 109.54 1.65*Soil moisture 2 1274.29 637.65 9.61t

Bound water Interaction 2 129.87 64.94 0.98*Error 54 3581.43 66.32Total 59 5095.13

Species 1 1920.6 1920.6 10.55 tSoil moisture 2 24202.3 12101.2 66.49t

Total water Interaction 2 187.1 93.6 0.51*Error 54 9827.2 182.0Total 59 36137.2

Species 1 66.03 66.03 51.19tSoil moisture 2 540.34 270.17 209.43t

sure Interaction 2 129.06 64.53 50.02 tError 54 69.57 1.29Total 59 805.00

Species 1 1.3735 1.3735 88.61 tSoil moisture 2 3.1324 1.5662 101.05tSolute .concen- Interaction 2 0.6717 0.3358 21.66ttration Error 54 0.8373 0.0155Soil moisture 2 24203.0 12101.0 66.49tTotal 59 6.0149

* Not significant.t Highly significant.

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PLANT PHYSIOLOGY

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FIG. 2. Mean values of bound water, total water, osmotic pressure, and solute con-

centration in leaves of loblolly pine and shortleaf pine under various soil moisture condi-tions. Each value is the mean of 10 determinations.

30 = Soil moisture maintained continuously at 30 per cent.W.C. = Soil moisture at the wilting coefficient.

W.C.-30 = Soil moisture at 30 per cent. after having beenreduced to the wilting coefficient.

454

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PLANT PHYSIOLOGY

pine supplied continuously with 30 per cent. soil moisture and. of the samespecies at 30 per cent. soil moisture after having been reduced to the wiltingcoefficient showed no significant difference. No comparison was made onthe transpiration rates of seedlings of shortleaf pine at these two soil mois-ture contents.

PHYSICO-CHEMICAL PROPERTIES OF LEAF TISSUES

Analyses of variance were made on all physico-chemical data and arepresented in table III. These analyses show that total water, osmotic pres-sure, and solute concentration in leaves of shortleaf pine and loblolly pinediffered significantly between species as well as between soil moisture cate-gories. The analyses also show that soil moisture treatments caused signifi-cant differences in bound water contents, and that species did not. Themean values of each physico-chemical property in each category are pre-sented graphically in figure 2. Tests of the significance of differencesbetween individual means are presented in table IV.

The bound water content values may be subject to criticism because ofthe large variation within categories as indicated by the magnitude of themean square of error in the analysis of variance in table III. Two factorsmay contribute to this variation: first, inconsistencies in the technique usedin making the determinations; and second, variation in the amount of boundwater from tree to tree within categories. No replicated bound water deter-minations were made on substances known to be homogeneous; hence anabsolute check on the reliability of the method is not available. The replica-tions of the specific heat determinations, however, and the calorimeter factordeterminations, presented in table V, are very consistent. Since these deter-

TABLE VCOMPARISON OF VARIATION IN REPLICATED DETERMINATIONS OF BOUND WATER, SPECIFIC HEAT,

AND THE CALORIMETER FACTOR

SPECIFICHEAT OF CALORIMETER BOUNDITEM LOBLOLLY FACTOR WATER*

PINE

Number of replications.5 4 10Mean.0.213 1.070 35.81Standard deviation.0.00336 0.00895 7.78Standard error.0.00151 0.00448 2.46Standard error expressed as a percentage

of the mean.0.7 0.4 6.9

* Determinations were made on leaves of loblolly pine when soil moisture was at 30per cent.

minations were made using the same apparatus that was used in making thebound water determinations, the consistency of the specific heat values andthe calorimeter factors is an indication of the reliability of the technique.

456




When soil moisture was at 30 per cent., leaves of shortleaf pine werehigher in total water, osmotic pressure, and solute concentration than wereleaves of loblolly pine. Bound water contents were not significantly differ-ent in leaves of the two species. Consideration of these data alone mightlead to the conclusion that the greater drought resistance of shortleaf pineis attributable to osmotic effects. Data on the physico-chemical propertiesunder the other two soil moisture conditions show, however, that osmoticeffects have no direct connection with the relative drought resistance of lob:lolly pine and shortleaf pine.

Changes in the seedlings which occurred when soil moisture was reducedfrom 30 per cent. to the wilting coefficient furnish some interesting informa-tion on the behavior of the two species during drought hardening. Seedlingsof both species became dormant as judged by the cessation of growth andthe setting of terminal buds. In leaves of both species osmotic pressure in-creased, and total water decreased considerably. Solute concentration ap-parently decreased in leaves of shortleaf pine, but increased in leaves of lob-lolly pine. These changes in solute concentration, although small, are statis-tically significant. They are not believed to be physiologically significant,however, because of the fact that the changes are in opposite directions inthe two species.

No significant change occurred in the bound water content of the leavesof either species when soil moisture was reduced from 30 per cent. to thewilting coefficient. This fact leads to the assumption that no appreciablechange took place in the amount of water-binding colloids and crystalloidsin leaves of the two species during the drought hardening period. The factthat no consistent change in solute concentration occurred in the two specieswhen soil moisture was reduced may be regarded as evidence partially sup-porting this assumption.

Physico-chemical differences between species when soil moisture was atthe wilting coefficient show which properties cannot be used for determiningthe relative drought resistance of the two species and also show which factorsmight be indicative of the relative drought resistance. With soil moisture atthe wilting coefficient, leaves of shortleaf pine had a higher total water con-tent and a lower osmotic pressure than leaves of loblolly pine. Solute con-centration and bound water contents were practically the same in bothspecies.

Since solute concentration on a dry weight basis was the same in the twospecies when soil moisture was at the wilting coefficient, it follows that theobserved difference in osmotic pressure was the result of the difference intotal water content. Since osmotic pressure was influenced in this way, itcannot be used as an indicator of the relative drought resistance of the twospecies.

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PLANT PHYSIOLOGY

The simultaneous occurrence of a higher total water content and a liighe rtranspiration rate in leaves of shortleaf pine when soil moisture was at thewilting coefficient, indicates that this species had a faster rate of absorptionof water from soil than loblolly pine when soil moisture was limited. Thisability of shortleaf pine might be attributed to a lower ratio of evaporatingleaf surface to absorbing root surface. A superficial examination of theroot systems of the seedlings used in this work indicated that shortleaf pineseedlings had a lower top-root ratio than loblolly pine. More exact mea-surements are necessary, however, to prove the point.

The ability of a seedling to maintain a higher total water content in itsleaves when soil moisture is limited may be an aid to survival during periodsof drought in that it might prolong the period of permanent wilting andthus delay the occurrence of a water content in the leaves below the minimumnecessary for life. Such a prolongation of the period of permanent wilting,however, would enable a seedling to survive periods of drought only as longas the soil moisture content did not go below the wilting percentage for aprolonged period.

The fact that shortleaf pine, the more drought resistant of the two speciesstudied, had the greater total water content in its leaves when soil moisturewas at the wilting coefficient, leads to the conclusion that the magnitude ofthe total water content may be used as an indicator of the relative droughtresistance of the two species under the conditions of this experiment.

In view of the correlation obtained by other workers between boundwater contents and relative drought resistance, an explanation of the lackof such correlation in this study, when soil moisture was at the wiltingcoefficient, is pertinent. NEWTON and MARTIN (17) who obtained a goodcorrelation between drought resistance and bound water contents of cerealcrops and grasses, used the cryoscopic method for the determination ofbound water in expressed sap. With this method, only the amount of boundwater held by the colloids, ions, and molecules expressed in the sap is deter-mined. MEYER (13) stated that a considerable amount of water may bebound in cell walls of plant tissues. Since leaves of many species of thegenus Pinus, including loblolly pine and shortleaf pine, are characterizedby a hypodermal layer of cells with thick walls, the amount of water bound'by dispersed substances within the cells may be negligible compared to theamount of water bound in the cell walls. In this investigation, bound waterdeterminations were made on macerated leaf tissue which included both cellwalls and cell contents. The amount of water-binding surface in cell wallsper unit weight of dry leaf tissue may have been sufficiently uniform in thetwo species to prevent the detection of small differences in the amount ofbound water in the cell sap. Since no data are available on the relativeamounts of bound water in cell walls and in cell sap, any interpretation of

458




the bounid water contents of the leaf tissue must be based on their face value.Hence, the magnitude of the bound water content of the leaf tissue as deter-mined in this investigation, is not a factor which makes shortleaf pine moredrought resistant than loblolly pine.

Striking changes occurred in the leaves of the two species after soilmoisture was increased from the wilting coefficient to 30 per cent. Theplhysico-chemical properties were determined one week after the soil mois-ture had been brought back to 30 per cent. During that week dormancywas broken and growth was resumed. Total water contents of leaves weregreater than when soil moisture was at the wilting coefficient, but less thanwhen seedlings were first supplied with 30 per cent. soil moisture. Largedecreases in bound water, osmotic pressure, and solute concentrationoccurred in leaves of both species. Each of these three properties werelower in magnitude than in either of the other two soil moisture categories.These results agree with those of VASSILIEv and VTASSILIEV (29) who foundthat wheat plants grown in soil with insufficient water and then wateredabundantly were lower in water content as well as in monosaccharides andsucrose than check plants grown continuously with abundant soil moisture.

Total water, osmotic pressure, and solute concentration were higher inleaves of shortleaf pine than in leaves of loblolly pine after 30 per cent. soilmoisture was restored. No significant difference in the bound water coi-tents of the two species was observed in this soil moisture category.

The decreased solute concentration and the decreased bound water con-tenlts in the leaves of the two species indicate a rapid depletion of food re-serves, including water-binding colloids, in the leaves when dormancy result-ing from drought is broken by the restoration of abundant soil moisture.The fact that the decrease in solute concentration was not as great in short-leaf pine as in loblolly pine indicates that the former species may have main-tained a better balance between utilization and synthesis of soluble food re-serves. This closer balance may enable shortleaf pine to recover morerapidly from the effects of drought, and hence it may be a factor contribut-ing to the ability of this species to survive on drier sites than those on whichloblolly pine occurs.4 Summary

Determinations of transpiration, bound water, total water, and osmoticpressure were made on seedlings of loblolly pine and shortleaf pine to obtaininformation on the behavior of these species during and after drought andto ascertain whether any of these factors can be used as indicators of relativedrought resistance. Solute concentration, calculated from data on osmoticpressure and total water content, was a valuable aid in interpreting theosmotic pressure differences.

With soil moisture at 30 per cent., shortleaf pine had a higher transpira

4'a9



PLANT PHYSIOLOGY

tion rate, more total water, a higher osmotic pressure, and a greater soluteconcentration than loblolly pine. Bound water contents were not signifi-cantly different in the two species.

With soil moisture at the wilting coefficient, shortleaf pine had a highertranspiration rate, more total water, and a lower osmotic pressure than lob-lolly pine. Solute concentration and bound water contents were practicallythe same in both species.

When soil moisture was restored to 30 per cent. after having been re-& duced to the wilting coefficient, leaves of shortleaf pine had more total water,

a greater osmotic pressure and a higher solute concentration than leaves ofloblolly pine. Transpiration rates and bound water contents were prac-tically the same in both species.

Bound water contents decreased in both species when soil moisture wasrestored to 30 per cent. after having been reduced to the wilting coefficient.

Points observed in this study which may contribute to the greaterdrought resistance of shortleaf pine compared to that of loblolly pine areas follows:

1. Shortleaf pine absorbed more water from the soil and at the same timemaintained a higher total water content in its leaves even when soil moisturewas limited.

2. Shortleaf pine maintained a higher solute concentration when recover-ing from the effects of drought; that is, when 30 per cent. soil moisture wasrestored.

The data also show that the greater drought resistance of shortleaf pinecannot be attributed to an ability to conserve water either by retardingtranspiration or by forming bound water. Further, the greater droughtresistance of this species cannot be attributed to a higher osmotic pressure.

NORTHERN ROCKY MOUNTAIN FORESTAND RANGE EXPERIMENT STATION

MISSOULA, MONTANA

LITERATURE CITED

1. BATES, C. G. Physiological requirements of Rocky Mountain trees.Jour. Agr. Res. 24: 97-164. 1923.

2. DOLE, E. J. Studies on the effects of air temperature and relativehumidity on the transpiration of Pinus strobus. Vermont Agr.Exp. Sta. Bull. 238. 1924.

3. DRABBLE, E., and DRABBLE, HInDA. The relation between the osmoticstrength of cell sap in plants and their physical environment. Bio-chem. Jour. 2: 117-132. 1907.

4. GAIL, F. W., and LONG, E. M. A study of site, root development, and

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transpiration in relation to the distribution of Pinus contorta.Ecology 16: 88-100. 1935.

5. GREATHOUSE, G. A. Unfreezable and freezable water equilibrium inplant tissues as influenced by subzero temperatures. PlantPhysiol. 10: 781-788. 1935.

6. HARRIS, J. A., and GORTNER, R. A. Notes on the calculation of theosmotic pressure of expressed vegetable saps from the depressionof the freezing point with tables for the values of P for A = 0.0010to A=2.9990. Amer. Jour. Bot. 1: 75-78. 1914.

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